The present invention relates to a process for selective hydrogenation of C10-C16 diolefins in the product from the dehydrogenation of C10-C16 paraffins to mono-olefins. More particularly, it relates to a process which can simultaneously enhance the conversion and selectivity of selective hydrogenation of C10-C16 diolefins under mild conditions by using a combination of a specific catalyst with a multi-stage hydrogenation mode
The olefin stream produced in the plant for producing the feedstock of the is detergent-linear alkyl benzene by dehydrogenation of C10-C16 paraffins contains about 1-3 wt % of diolefins. The presence of these diolefins will cause a lot of side reactions in the subsequent alkylation, resulting in a decrease of the yield and quality of alkyl benzene. The quality of alkyl benzene may be effectively improved on the basis of enhancing the yield of alkyl benzene by selective hydrogenation of diolefins in the dehydrogenation product to mono-olefins.
U.S. Pat. Nos. 4,695,560, 4,523,048, 4,520,214, 4,761,509 and Chinese Patent CN 1032157 disclose a process for selective hydrogenation of diolefins in the product from dehydrogenation of C8-C20 paraffins. The characteristics of the catalyst in this process are that it contains 1.0-25 wt % of nickel, 0.05-1.5 wt % of sulfur and the supporter is small Al2O3 balls made by the oil-drop method, which balls have a pore volume of 1.44-3.0 cm3/g, a surface area larger than 150 m2/g and have no precious metals, and essentially have no halogens, alkali earth metals and alkali metals ( less than 0.1 wt %). Because the main active element of the catalyst used in this process is nickel, selective hydrogenation has to be conducted at a temperature higher than 200xc2x0 C. to attain a certain activity. This would results in the occurrence of the side reaction-cracking and the increase of the consumption of the feed material. Meanwhile, because this process uses a single-stage reaction mode and hydrogen needed by hydrogenation is introduced at one time, the reaction pressure must be retained at above 1.1 MPa to allow hydrogen to fully dissolve in the liquid mixture of paraffins and olefins In addition, in order to increase the selectivity of diolefins, to mono-olefins, it is necessary to frequently sulfurize the catalyst so as to suppress its activity. Sulfurization is a complicated technology and it is difficult to control the amount of sulfur injecting onto the catalyst in operation. If this amount is too large, the activity of the catalyst is very low, while if this amount is insufficient, the selectivity of the catalyst is very poor. Furthermore, the investment will increase due to the equipment of the sulfur-injecting system.
Chinese patent CN 1236333A reports a process for preparing a selective hydrogenation catalyst and its applicable scope, which catalyst contains palladium and at least one element selected from tin and lead. Alumina with a specific surface area of 5-200 m2/g and a pore volume of 0.3-0.95 cm3/g is used as the supporter of the catalyst reported in this patent and at least 80% of the active element, palladium, is distributed within the volume between the surface and the spherical face at the depth of 500 xcexcm of the catalyst particles by impregnation. The promoter elements selected for enhancing the selectivity of the reaction are tin and lead. The catalyst is suitable for selective hydrogenation of lower hydrocarbons such as butadiene, etc., but does not suit selective hydrogenation of C10-C16 long chain diolefins because both its specific surface area and pore volume are quite small.
U.S. Pat. No. 4,704,492 reports a multi-stage selective hydrogenation process for removing acetylenic impurities from a natural butadiene gas. The characteristic of this process is to effectively decrease the ratio of hydrogen to the compounds to be hydrogenated by separately injecting hydrogen into each stage and thereby effectively enhance the selectivity of hydrogenation.
The objective of the present invention is to effectively remove diolefins, which are the by-products simultaneously generated in dehydrogenation of C10-C16 paraffins for producing mono-olefins and avoid the shortcomings of the requirements for high temperature and pressure, frequent sulfurization of the hydrogenation catalyst existing in the prior arts which use nickel-containing catalysts.
In order to adapt the requirement of the reaction of C10-C16 diolefins, xcex3-alumina with a specific surface area of 50-300 m2/g, a pore volume of 0.2-2.0 cm3/g, is used as a supporter, which allows the long chain mono-olefins to rapidly diffuse in the pores of the catalyst and permits the use of the catalyst in selective hydrogenation of C10-C16 diolefins. The reaction temperature and pressure can be lowered to a large extent because the activity of palladium-containing catalysts is much higher than that of nickel-containing catalysts. Because a multi-stage hydrogenation process is used, the selectivity and conversion of diolefins to mono-olefins is increased by controlling the hydrogen/diolefin molar ratio and the decrease of the yield due to hydrogenation of mono-olefins to paraffins is avoided, thereby the yield of mono-olefins is increased. The shortcomings of complicated and unstable operation and high investment, etc., resulting from the requirement of the nickel-containing catalysts for sulfurization were avoided as no sulfurization is needed for enhancing the selectivity of diolefins to mono-olefins. Through combining the above several approaches and applying them to selective hydrogenation of C10-C16 diolefins, the selectivity and conversion of C10-C16 diolefins to mono-olefins are increased.
The present invention provides an improved selective hydrogenation process for removing C10-C16 diolefins in the product from the dehydrogenation of C10-C16 paraffins to mono-olefins, which process includes bringing the mixture stream of paraffins and olefins containing C10-C16 mono-olefins and C10-C16 diolefins into contact with a specific hydrogenation catalyst in a plurality of hydrogenation reactors connected in series under the reaction conditions for hydrogenation The inlet of each reactor has a hydrogen injection pipe respectively and hydrogen is injected into each reactor. To convert diolefins in the mixture stream of paraffins and olefins into mono-olefins by hydrogenation, xcex3-alumina having a specific surface area of 50-300 m2/g and a pore volume of 0.2-2.0 cm3/g is used as the supporter of the hydrogenation catalyst palladium being supported or the supporter as the main catalyst element and an element selected from silver, gold, tin, lead or potassium being supported on the supporter as the promoter.